Centrifugal fans are used for ventilation in air cleaners, air conditioners, and the like. FIG. 1 and FIG. 2 depict one conventional example of a centrifugal fan called a multi blade fan. Further, FIG. 1 depicts a side view (specifically, a cross sectional view taken along A—A in FIG. 2) of a conventional example of a multi blade fan, and FIG. 2 depicts a plan view of a conventional example of a multi blade fan.
A multi blade fan 10 comprises an impeller 13, a housing 11 that houses the impeller 13, a motor 14 for rotating the impeller 13, and the like. Further, the axis O—O in FIG. 1 and FIG. 2 is the rotational axis line of the impeller 13 and the motor 14.
One end of each of numerous blades 33 (only some of the numerous blades 33 are illustrated in FIG. 2) of the impeller 13 are fixed to the outer circumferential edge of a discoidal main plate 31, and the other end of each of these blades 33 are connected by an annular side plate 32.
The housing 11 is a scroll shaped box body when viewed from a plan perspective, and comprises an opening 11a and a gas outlet 11b. 
A bell mouth 12 is arranged so that it covers the opening 11a of the housing 11, and an inlet 12a is formed therein for guiding the inlet gas to the impeller 13. The inlet 12a is arranged so that it opposes the side plate 32 of the impeller 13. The bell mouth 12 comprises a curved part 12b that extends on inner circumferential edge of the inlet 12a toward the impeller 13 side, and a flat part 12c formed so that it covers the opening 11a on an outer circumferential side of the curved part 12b in the radial direction and that extends in a direction that intersects a rotational axis line O—O.
If the multi blade fan 10 is operated by driving the motor 14, then the impeller 13 rotates oriented in the rotational direction R in FIG. 2 with respect to the housing 11. Thereby, all of the blades 33 of the impeller 13 raise the pressure of and blow out the gas from the space on the inner circumferential side to the space on the outer circumferential side, take gas from the inlet 12a into the space on the inner circumferential side of the impeller 13, and collect and blow out to the outlet 11b the gas that was blown out to the outer circumferential side of the impeller 13. In other words, as shown by the arrow W depicted in FIG. 1 and FIG. 2, the multi blade fan 10 principally takes in gas from the rotational axis line O—O direction and blows out gas from the outlet 11b. 
Patent Document 1
Japanese Published Patent Application No. H09-209994